The lords were the at the top of the social class system. They controlled everything. They lived in the castle with their family. They also provided protection to the serfs
Serfs were at the bottom of the social class system. They were considered peasants. They were not slaves because they could not be bought or sold. They were given land in exchange for work. They were right below the knights
Knights were right under the priest and nobles on the social class system. They provided protection for everybody. In return they received land and certain privileges
Priest and nobles were right under the kings and lords. But in some cases the priest and nobles had more power. They were in charge of the knights. That is why in some cases kings/lords ordered an army.